In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system performs these functions using field devices distributed at key locations in the industrial process and coupled to the control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices currently known, or yet to be known, used in the measurement, control and monitoring of industrial processes. Usually, such field devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and be able to withstand climatological extremes of temperature, humidity, vibration, and mechanical shock. Field devices typically operate on relatively low power. For example, some field devices are currently available that receive all of their operating power from a known 4-20 mA loop operating at relatively low voltages (12-42 VDC).
Some field devices include a transducer. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and others.
Typically, each field device also includes communication circuitry that is used for communicating with a process control room over the process control loop. In some installations, the process control loop is also used to deliver a regulated current and/or voltage to the field device for powering the field device. The process control loop also carries data, either in an analog or digital form.
Traditionally, analog field devices have been connected to the control room by two-wire process control current loops, with each device connected to the control by a single two-wire control loop. A voltage differential is maintained between the two wires within a range of voltages from 12-45 volts for analog and 9-50 volts for digital mode. Some analog field devices transmit a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. Other analog field devices can perform an action under the control of the control room based on the magnitude of the current through the loop set by the control room. In addition, or in the alternative, the process control loop can carry digital signals for communication with field devices.
The environment within which the field devices operate are often made hazardous by the presence of flammable or combustible materials. In these areas, a spark or high surface temperature of a component could cause local atmosphere to ignite and propagate an explosion. These areas are referred to as Hazardous (Classified) areas. As a method of preventing unwanted ignitions, intrinsic safety specifications have been developed as a means of ensuring limited energy and temperature in field devices. Compliance with an intrinsic safety requirement helps ensure that even under fault conditions, the circuitry or device itself cannot itself ignite a volatile environment. One example of an intrinsic safety requirement is set forth in: APPROVAL STANDARD INTRINSICALLY SAFE APPARATUS AND ASSOCIATED APPARATUS FOR USE IN CLASS I, II AND III DIVISION 1 HAZARDOUS (CLASSIFIED) LOCATIONS, CLASS 3610, promulgated by Factory Mutual Research, October, 1988. Additional or alternate approvals are also set forth in industrial standards such as Canadian Standards Association (CSA) and the European Cenelec Standards.
One technique that can be used to comply with intrinsic safety standards is to separate components with a physical barrier. The amount of separation is dependent on the specific material used to form the physical barrier, and the voltage that the barrier must withstand. Optical isolators are known to be used for transmitting data across the barrier if they are separated in accordance with intrinsic safety standards.
An optical isolator, also known as an optoisolator or optocoupler, is an electrical arrangement that transfers electrical signals between two isolated circuits using different frequencies of the electromagnetic spectrum. The isolation between circuits prevents large voltages or current induced or otherwise present in one of the circuits from being transferred or coupled to the other circuit. Typically, optoisolators cannot transfer measurable power between the circuits, but can convey signals between the isolated circuits.